Pulse oximetry has become an accepted method of oxygen determination in the last five years. However, up to this point, all devices on the market have used the transmission method of detection. The pulse oximeter of the present invention is based upon a reflectance method. Both of these methods are based upon several related facts.
First, the concentration of blood in a given location of the body varies with each pulse of the heart. This variation can be measured by optical methods by introducing a light source near the skin and detecting either the reflected or the transmitted light intensity. This intensity is directly related to the localized blood concentration.
Secondly, the wavelength of the light source determines the effect that oxygen saturation has on the reflected or transmitted intensity. The wavelength which does not change intensity with saturation but only with concentration is called the isobestic wavelength. This isobestic condition occurs at several wavelengths. By using this wavelength as a reference and by comparing it to a second wavelength in the red portion of the spectrum, it is possible to determine the oxygen saturation of the blood non-invasively.
Current pulse oximeters using the transmissive method require the light from the emitters to pass through the tissue to the detector on the opposite side. This requires the sensor to be placed on an area of the body where the distance from the emitters to the detectors is fairly small (about an inch at most) and not obstructed by opaque tissue layers, such as bone. This limits the placement of the sensors to areas such as the finger tips, ear lobes, or the bridge of the nose.